1. Field of the Invention
The present invention relates to a method of refining steel, particularly, decarburizing various steels such as low carbon steel, carbon steel, low alloyed steel, alloyed steel and stainless steel. The invention encompasses an apparatus for practicing the refining method.
2. State of the Art
As is well known, VOD process (vacuum oxygen decarburization) and AOD process (argon oxygen decarburization) are often used for refining various steels, particularly, stainless steel containing Cr.
AOD process is the most typical method of refining, which comprises injecting Ar gas with oxygen gas into molten steel to conduct decarbuzization while suppressing oxidation loss of Cr by lowering CO partial pressure, and, as the decarburization proceeds from the high carbon state to the low carbon state, changing the O.sub.2 /Ar ratio of the gas injected into the molten steel so that the decarburization proceeds efficiently. As the gas for diluting O.sub.2, instead of expensive Ar, N.sub.2 may be used, or alternatively, steam may be used (CLU process).
The AOD process is conducted without additional heat to the molten steel, and therefore, it is usual to start the refining at a high carbon state such as 1.0-2.0 weight % in the molten steel so as to utilize the heat of oxidatin reaction of the carbon and to prevent temperature decrease during the refining.
In such a process, the period for the decarburization becomes necessarily long. Also, it is inevitable that a portion of Cr is oxidized, and the oxide must be reduced afterward by using expensive metallic Si. Further, it is not easy to control temperature of the molten steel to be tapped. These are the problems inherent in AOD process.